Mavirus (Maverick Virus)
A Microbial Biorealm page on the genus Mavirus
Mavirus virophage (not currently classified under the ICTV or Baltimore system)
Description and Significance
The Mavirus, short for ‘Maverick Virus’, is a newly discovered dsDNA virophage; meaning that it is a virus whose replication cycle is fundamentally dependent on another ‘helper virus’, which enables replication (1). Mavirus was discovered within Cafeteria roenbergensis, a single-celled eukaryotic marine zooplankton that until recently was thought to only host the CroV virus of the Mimiviridae family (2). As a true virophage, Mavirus cannot replicate without CroV present in Cafeteria roenbergensis (1). While relatively miniscule to the overwhelmingly large CroV virus (at 700,000 base pairs, this makes it the marine largest virus yet discovered), the Mavirus utilizes the cell replication machinery encoded for by CroV, and ultimately limits CroV infection (7). Furthermore, the Mavirus is hypothesized to be an evolutionary link between transposons of viruses and eukaryotes, thus having influencing the evolution of many eukaryotic genomes (8).
While the Mavirus is highly complex in its ecological function, its genome consists of merely 19,063 base pairs, encoding twenty genes (7). Among these twenty genes, a dsDNA retroviral-integrase transposon intermediate, putative capsid proteins, and a protein-primed DNA polymerase B have been confirmed (5). Furthermore, of these twenty genes, seven transposons found in contemporary eukaryotes of the Maverick/Polinton family have been matched to the genome of the Mavirus; however, these transposons do not tend to move themselves into the genome of other virophages, but rather those of eukaryotes (6). It is also thought that current eukaryotic dsDNA transposons are derived from vestigial genomic information of ancient dsDNA viruses; in other words, the ancestors of the Mavirus allowed their transposons to be integrated into the ancestors of contemporary eukaryotes, and the Mavirus represents that missing evolutionary link (6,7). Because transposons work as ‘self-jumping genes’, which are capable of integrating themselves into different genomes, they can influence the evolution of species other than their original holder. Therefore, it has been hypothesized that the genome of the Mavirus, and those of other virophages (some yet to be discovered) will correspond to most major families of transposons (6).
Structure, ‘Metabolism’ & Life Cycle
As previously stated, the replication cycle of the Mavirus requires the CroV virus within Cafeteria roenbergensis in order to proceed (1). As an aside, the CroV virus is among the largest viruses ever discovered with approximately 544 predicted protein-coding genes (and expressing 274 in infection); as such, it provides many diverse metabolism enzymes that can be utilized by the Mavirus (9).
Ecology (including pathogenesis)
Currently, the Mavirus only infects Cafeteria roenbergensis that are being infected by the CroV virus, though as previously mentioned, transposable elements of its genome can be incorporated into eukaryotes (1,3,6,7). The specific mechanisms that enable the replication of the Mavirus within the CroV-infected Cafeteria roenbergensis are currently unknown. However, the Mavirus exists in a marine environment, and whose presence has been closely linked with the CroV virus, due to the compatible replication machinery, and energy harnessing capabilities (1).
The recent discoveries of the Mavirus, and other similar virophages has re-sparked a vigorous debate by taxonomists regarding whether viruses (and particularly, large polycistronic virophages) can be considered living, and thus add a whole other domain branch to the tree of life. Two major points converge here: Insights into the genetic code of the virophage ‘Sputnik’ has shown that certain genes from its virophage constituents have been absorbed over its ‘evolutionary’ history. The suggesting that some form of horizontal gene transfer can occur between different virophages (4). And, while ‘regular’ (non-virophage) viruses use membrane-protein to exploit living cells for the purpose of reproduction, virophages undergo the same process with one exception—the way they exploit the host virus’ replication machinery is argued to be synonymous to the way any other parasite would exploit its host (3). While some scientists consider this tactic to be a new riff of the same theme, it is impossible to ignore the implications of these findings; as time passes, we are learning that pathogenic viruses more closely resemble bacteria and other ‘living’ organisms more so than we ever could have anticipated.
1) http://watchingthewatchers.org/indepth/1388036/virophage-virus-eater Date Accessed: 9/4/2011
2) http://eol.org/pages/912371/details Date Accessed: 10/27/2011
3) http://www.nature.com/news/2008/080806/full/454677a.html Date Accessed: 10/27/2011
4) http://www.ncbi.nlm.nih.gov/pubmed/17034960 Date Accessed: 10/28/2011
5) http://www.ncbi.nlm.nih.gov/pubmed/21385722 Date Accessed: 10/28/2011
6) http://www.genomeweb.com/new-virophage-points-evolutionary-relationship-between-transposons-and-dna-virus Date Accessed: 10/28/2011
7) http://arstechnica.com/science/news/2011/03/a-virus-so-large-it-gets-viruses.ars Date Accessed: 10/29/2011
8) http://www.sciencemag.org/content/332/6026/231.short Date Accessed: 10/29/2011
9) http://www.pnas.org/content/early/2010/10/15/1007615107 Date Accessed: 10/30/2011